STATE GEOLOGIST. 19 



f 



very closely resemble the Huron county gritstones, and are 

 regarded by Dr. Newberry as the base of the Portage Group 

 in Ohio. 



Further down the ravine are seen twenty or thirty feet of 

 dark fissile shales, covered with iron rust and an astringent 

 efflorescence, and in every respect resembling the shales whichr 

 underlie the gritstones of Lake Huron. Unfortunately there is 

 no possibility of founding an equivalency on palaiontological 

 evidence. Aside from this I am constrained to regard the flag- 

 stones and shales of Cleveland as on the horizon of the grit- 

 stones and shales of Lake Huron. Bat the Cleveland shales 

 are regarded by Dr. Newberry as " Hamilton shales,'' perhaps, 

 however, using the term Hamilton in the extended sense, so as 

 to include all the New York strata from the Marcellus to the 

 Portage. If the overlying shales and flagstones of Lake Hu- 

 ron, and the underlying argillaceous limestones of Partridge 

 Pt. fall into the Hamilton Group, the intermediate black bitu- 

 minous shales occupy the same position. So I had been in- 

 clined to regard them So I subsequently learned the black 

 shales of Enniskillen were at first regarded by Mr. Billings, 

 though he afterwards placed them in the Portage Group on the 

 judgment of Prof. Hall. This palaaontologist, whose authority 

 is not to be questioned where palasontological evidence is 

 within reach, thinks he likewise recognizes in the vegetable 

 impressions of the black shales of Michigan, and in their gen- 

 eral physical characters, satisfactory affinities with some of 

 the shales of the Portage Group. In this state of the case we 

 shall be constrained for the present to regard the Huron 

 Group of Michigan, extending from the conglomerate above 

 the gritstones of Huron county, -to the top of the argillaceous 

 limestones of Partridge Pt., as probably representing the rocks 

 of, the Portage Group of New York. 



From the description which has been given of the Huron 

 Group in its northern and southern outcrops, it appears that 

 the group is composed of coarser materials' toward the north, 

 and probably attains in that direction, much the thickest devel- 



